1. Field of the Invention
The present invention relates to a slackened needle thread assuring unit capable of drawing out the needle thread necessary for a stitching operation.
2. Description of Prior Art
A known sewing machine is provided with a cutter for cutting a needle thread and a bobbin thread and with a thread end holder.
The cutter for cutting the needle thread and the bobbin thread permits a needle to be positioned at an upper stop position when the needle is raised from the surface of the sewn cloth after completion of the stitching operation and also permits a knife of the cutter to automatically cut off the needle and the bobbin threads at the position directly under the sewn cloth, namely, under a throat plate on which the sewn cloth is placed. The machine provided with the cutter is called a thread end cutting sewing machine. The thread end holder clamps the end portion of the needle thread which is cut off by the cutter to assure proper formation of a first stitch on the cloth to be sewn.
However, the prior art sewing machine is subject to a thread end breakage problem which can occur when the sewing operation is started from the state where by needle thread and the bobbin thread are cut off short. Then, the tension of the needle thread is abnormally increased and the end of the lower thread is drawn over the upper surface of the sewn cloth whereby the first stitch is not formed by a first needle hole, and in the worst case, the needle thread is cut off. This occurs because the turning angle of the upper shaft of the sewing machine increases to 70 degrees. Because of the structure of the sewing machine, the distance between a hole of the needle and a thread hole of the thread takeup is enlarged, causing a shortage in the slack of the needle thread when the needle is moved from the upper stop position to a lowering step at the start of the sewing operation. As a result, as manual finishing process is required after completion of the sewing operation. An end breakage can occur when an excessive tension is applied to the portion of the thread extended between the thread tension member and the needle hole via the thread takeup.